testing for storage location of static const member

Question

I have a

class A  
{  
private:  
    static const int b = 10;  
public:
    static void testReadOnly(int _b)
    {
        const_cast<int &>(A::b) = _b;
    }
};  

and I want to test whether the member variable b is initialized at compile time and therefore stored in the code-segment (read-only).
To do so, I try to change the value of b, which should produce some sort of runtime error (i.e. Segmentation fault, thrown by the MMU), if it actually is stored in the code-segment.

I supposed that above code should build, but my compiler/linker tells me undefined reference to 'A::b'

Why?

Answer 1

Put a definition for the static member, outside of class declaration to solve linkage errors:

class A  
{  
    static const int b = 10;
    ...
};

const int A::b;
~~~~~~~~~~~~~~~

In addition, any modification of a constant value (by weird castings) will invoke undefined behavior.

Undefined behavior is a unknown behavior, sometimes causes to crash the application, sometimes not.